Volatile liquids such as crude oil and other refined petroleum products must normally be stored for a period of time after recovery or after being produced. As a matter of practicality the liquid is either piped or carried to a tank farm or a tank battery. Thereafter it is gradually withdrawn as needed either to be processed further, stored, or shipped elsewhere.
In any instance, the stored volatile product will normally cause a vapor accumulation beneath the tank top. Depending on what the stored product is, these vapors can be dangerous to the atmosphere, or even constitute an explosive mixture.
As an example, vapor recovery systems used to gather and compress rich gas vapors from crude oil stock tanks have been in existence for many years. Such systems generally bleed off vapors, at near atmosphere pressure, compress them, and pass them on. These systems are, however, generally designed with a vapor removing compressor of sufficient capacity to handle a predetermined volume of gas which emanates from the stored liquid.
Normally the vapor compressor takes its suction from a storage tank vapor line and then discharges the compressed gas either into a sales point or other gathering system for further handling. While it is relatively simple to handle large volumes of stored liquid and consequently large volumes of vapor, the same does not hold for handling smaller volumes.
Cost and technical factors constitute one reason why many operators avoid handling smaller volumes of gas and liquid. However, energy conservation considerations, as well as gas pricing factors, coupled with various environmental regulations have altered this philosophy. Today, producers must reconsider the retention of these rich vapors that are normally lost to the atmosphere or are merely burned to be disposed of.
The most pertinent problems that arise in most known vapor recovery units are due to changing or variable gas volumes. Compressors which are incorporated into the system generally stop and start, or bypass the gas at such times as suitable pressure sensitive switches indicate low or high volumes. Further, volume surges of vapor will present problems to a compressor operation which is adapted or built for a relatively even vapor flow.
If, for example, the compressor is of a sufficiently large capacity for gas surges, that is, when a large volume of vapor is being generated, it must bypass gas and alternately shut down between such surges. However, bypassing of the gas around the compressor for any length of time is an inefficient operation. It can for example cause damage to the compressor if the unit is operated in this manner over long periods of time without adequate protection.
If the compressor shuts down, and must soon be started up again, the electrical starting contacts tend to rapid deterioration from such abusive use since this can exceed normally designed contact life. Added to this, the tank batteries when full have very small vapor space, a factor which further aggravates the compressor's stop-start system as well as the compressor's bypass system.
To compound these problems, in some instances rapid evacuation of storage tank liquids to a pipeline starves the compressor and can actually cause an ingestion of air into the tanks through the vacuum breakers. This air is then pumped into the gas system.
To rectify this latter problem, pressure reducing regulators are utilized to inject gas in the system. Consequently, a series of low pressure control systems must be delicately balanced and maintained at all times to achieve any semblance of continuous efficient operation.
It is therefore desirable to provide means for safely and effectively receiving and holding vapors from a volatile contained liquid. It is appreciated, however, that such accumulations of vapor will, if contained, build to excessive pressures. To assure only a minimal vapor pressure build-up, a flexible walled or expandable reservoir is provided in which to receive these vapors which would otherwise be forced into the atmosphere by way of a venting means. The expandable receptacle or reservoir functions in conjunction with a compressor and a controller, whereby to periodically activate the compressor to remove a predetermined amount of vapor.
It is therefore an object of the invention to provide storage means for a volatile liquid which avoids passage of harmful vapors into the atmosphere.
A further object is to provide storage and transfer means for holding both liquid and vaporous material, the latter being held in such manner that venting of the vapors is avoided.
A still further object is to provide a system for holding a volatile liquid in which an expandable walled reservoir or container is provided to retain vapors until the latter are disposed of by periodically actuating a compressor which is so connected to withdraw said vapors.